The present invention relates broadly to a modulation apparatus, and in particular to a pulse repetition modulation apparatus for the Improved Emergency Rocket Communication System (IERCS).
It will be well recognized that the possibility of enemy high altitude nuclear warhead detonations for purposes of land based missile pin-down becomes an almost certainty when the effects of this class of attack on communication is also considered. A series of studies of the relative efficiency of voice signals compared to digital signals, readily identified that for an EAM type message (where a limited number of alphanumerics are to be transmitted repetitiously and message piecing is permitted) a digital message is far more robust in a nuclear disturbed environment. The possibility of the modification of one of the many type of AM voice receivers proliferated with the armed forces was investigated by GE and Magnavox and it was concluded that the current AN/ARC-164 radio could be readily modified. The preferred alternative, in spite of the nuclear problem is to employ a voice transmitter in the IERCS vehicle. In view of the above, the following criteria were identified for this voice amplifier:
It must be added to DSCS-III Transpoder in the IERCS concept;
It must function with existing unmodified military UHF AM voice receivers;
It must provide a robust signal in a nuclear environment;
It must not add weight to IERCS that would degrade coverage/time performance.
The voice amplifier would be required to function over the increased ranges of the IERCS (as compared to the current ERCS) with sufficient power to be detected via the belly mounted antenna of a strategic tanker (as an example) into the whole spectrum of AM military voice radios. Thus, the link parameters place one major set of constraints on the voice amplifier design. The above noted voice radio must fit into the IERCS vehicle with minimum impact on needed power, (battery weight) needed cooling, (structure and/or cooling mechanism) and needed volume.
The state of the art of radio voice transmission is well represented and alleviated to some degree by the prior art apparatus and approaches which are contained in the following U.S. patents:
U.S. Pat. No. 2,430,139 issued to Peterson on Nov. 4, 1947; PA0 U.S. Pat. No. 2,568,721 issued to Deloraine et al on Sept. 25, 1951; PA0 U.S. Pat. No. 3,045,071 issued to Matthews et al on July 17, 1962; PA0 U.S. Pat. No. 3,534,728 issued to Barrows on Oct. 20, 1967; and PA0 U.S. Pat. No. 4,401,977 issued to Taylor on Aug. 30, 1983.
The Taylor reference discloses a radio transmitter which transmits bursts of oscillation at a preset carrier frequency for a preset number of carrier frequency cycles in each burst. The patent provides a relatively simple system in which the transmitter is controlled by a logic circuit that is driven by a counter which counts the number of oscillations in a transmission burst. The Peterson reference illustrates a communication system which utilizes spaced pulses of substantially equal length. The number of pulses which are transmitted varies with respect to the useful modulation. The Deloraine et al reference shows a communication system which utilizes an impulse modulation transmission system in which the impulses are transmitted at quite definite fixed moments and have a constant amplitude. Since the length of time during which the receiving circuit is in condition to receive can be reduced down to the duration of the impulses, thus the time during which interferences can act is considerably lessened. The Matthews et al reference is concerned with the problem of transmitting information over lines which are normally used for speech transmission. This reference discloses an apparatus for converting information that is represented by a direct voltage of varying magnitude into an alternating voltage having a frequency representative of the amplitude of the direct voltage. The invention comprises a saw-tooth generator for producing a saw-tooth signal, the amplitude of which increases from a datum at a predetermined rate and a comparator means arranged to compare the voltage to be converted with the saw-tooth generator output voltage and to cut-off and re-start the saw-tooth generator when the compared voltages are in predetermined relationship, whereby the saw-tooth generator produces a saw-tooth output that has a repetition frequency which is dependent on the voltage level that is to be converted. The Barrows reference presents a physiological parameter measuring system which employs a single blocking oscillator whose pulse repetition frequency is modulated by the parameters that are to be measured. However, none of the references disclose a voice link compatable with existing Air Force radios for penetrating nuclear environments. The present invention is intended to satisfy that need.